The aim of this work will be to investigate the properties of messenger RNA (mRNA) and heterogeneous nuclear RNA (HnRNA) in cultured cells (mouse L-cells). There will be four general categories of experiments: 1) The size and intramolecular location of repeated nucleotide sequences in mRNA and HnRNA will be investigated in the light of the hypothesis that these sequences represent untranslated regions involved in intranuclear processing, transport to the cytoplasm, or control of translation or degradation. 2) The relationships between non-repeated nucleotide sequences in poly A-containing (poly A(plus)) mRNA and HnRNA and poly A-lacking (poly A(minus)) HnRNA will be studied in order to determine what fraction of HnRNA nucleotide sequences is found in mRNA, and whether poly A (minus) HnRNA is a precursor of poly A(plus) HnRNA, or whether it represents an entirely different population of molecules. 3) To circumvent the nucleotide pool problems which complicate the study of RNA metabolism in animal cells, an approach will be explored for making cells dependent on exogenous RNA precursors. 4) These studies will be extended to a differentiated cell type making predominantly a single gene product that can be readily assayed.